1. Field of the Invention
The present invention relates to a device for protecting electronic semiconductor circuits against transients which occur on the supply line.
2. Prior Art
It is known to protect electronic circuits against transients, such as voltage peaks, which occur on the supply line by means of a Zener diode connected across the power supply to limit the voltage to a value which is not dangerous for the supplied circuits. At the end of the transient, the voltage after the Zener diode automatically returns to its normal value.
This protection, termed of the "ZENER" type, has the advantage of triggering itself automatically, of maintaining the operation of the supplied circuits during the transient, and of automatically restoring the normal supply conditions at the end of the transient. On the other hand, it has the disadvantage that the Zener diode must dissipate an instantaneous power which can be very high, high enough to burn it out, and it furthermore has the disadvantage that the non-zero dynamic impedance of the Zener diode allows the limited voltage to reach, for high values of the current in the Zener diode, levels which can be much higher than the breakdown voltage, with possible damage to the supplied circuits.
Another kind of protection, termed of the "CROWBAR" type, instead discharges the energy of the transient to the ground, short-circuiting the supply voltage. This protection is typically provided by means of an SCR or a TRIAC, triggered either by the voltage itself or by an impulse applied to their control electrode by a control circuit. As is known, once they are triggered, these components do not switch off until the voltage across them inverts or the current which flows through them drops below a limit value. To avoid the flow of an excessive current across the TRIAC or SCR for a long time, this kind of protection usually also interrupts a fuse arranged in series on the supply.
The CROWBAR type of protection therefore offers a high degree of safety even for high or prolonged transients, but it has other disadvantages, in particular difficulty in switching off, interruption of the operation of the supplied circuit even for transients with low energy content, and need to replace the fuse after intervention of the protection.